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Solution


Q.

As shown in the figure, a combination of a thin plano concave lens and a thin plano convex lens is used to image an object placed at infinity. The radius of curvature of both the lenses is 30 cm and refractive index of the material for both the lenses is 1.75. Both the lenses are placed at distance of 40 cm from each other. Due to the combination, the image of the object is formed at distance x = ____ cm, from concave lens.                     [2023]

Ans.

(120)

1f1=(1.75-1)(-130) 

⇒ f1=-40 cm

1f2=(1.75-1)(130) ⇒ f2=40 cm

Image from L1 will be virtual and on the left of L1 at focal length 40 cm. So the object for L2 will be 80 cm from L2 which is 2f. Final image is formed at 80 cm from L2 on the right.

So x=120